Hey i m no expert. I would wager the young are more informed and they know that the EU is the least of their problems (In fact, less than a problem). Or their young age allowed them to feel less desperate about their prospects.
You could easily flip that around and say that older people have the more informed opinion in addition to more life experience.
This comment is emblematic of the point Greenwald is trying to make here. If only those old people had more of the correct facts and the better answers, then they would line up behind the best decisions.